ES2686275T3 - An apparatus for reproducing a multichannel audio signal and a method for producing a multichannel audio signal - Google Patents

Abstract

A method of producing a multichannel audio signal from one or more sound object signals, comprising: For each sound object signal: Producing a plurality of decorrelated width signals from the duplicating sound object signal. a part of the sound object signal; Process the plurality of width signals to produce a plurality of panoramic signals, each panoramic signal being correlated with at least one channel; For each channel in the audio signal, combine the panoramic signals of each sound object for that channel; characterized in that for each sound object signal, the plurality of decorrelated width signals is produced by applying gains to each of the duplicate signals, in which the applied gains follow a Gauss distribution.

Description

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DESCRIPTION

An apparatus for reproducing a multichannel audio signal and a method for producing a multichannel audio signal Field of the invention

The present invention relates to multichannel audio systems.

Background and prior art

Multichannel audio systems are distinguished from stereo audio systems by the number of audio information channels and the corresponding number of speakers used for playback. While stereo systems are characterized by two channels, common multichannel audio systems have 5 or more channels.

One of the objectives of multichannel audio systems is to provide a listener with the immersive experience of an orchestra conductor or an artist on stage.

An important factor for such an experience is the ability to produce a "sound stage" in which each object - for example musical instruments - within the sound produced is perceived by the listener as originating from a position. Sound engineers place each sound object, usually in a virtual position between two channels, when mixing a multichannel audio signal. The component of each sound object in the two channels is then determined using amplitude panning. When each channel is played by a corresponding speaker, the sound is perceived by the listener as originating from a location determined by the amplitude panning and the location of the speakers with respect to the listener.

Another important factor for such an experience is the sound pressure level (SPL) the system is capable of producing where the listener is placed. Similar live concerts and performances may involve SPL peaks above 120 dB.

Most multichannel audio systems have speakers located near the walls of a room, with the listener placed toward the center of the room. To provide a 120 dB SPL in the listener with such an arrangement, the SPL in most positions along the walls of the room itself is greater than 120 dB, which is not desirable in residential environments.

European Patent Publication EP0966172A2 describes a method of synthesizing an audio signal having left and right channels.

Chang Ji-Ho et al, "Experimental validation of sound field control with a circular double-array of loudspeakers", Journal of the Acoustic Society of America, Vol. 133, No. 4, April 2013, p2046-2054 describes a Sound field control method with a set of double circular layer of speakers.

Ville Pulkki, "Uniform spreading of amplitude panned virtual sources", Proceedings of the 1999 IEEE Workshop on Audio and Acoustic Signal Processing Applications, 1999, p 187-190, describes a method for making directional propagation of panned sources of amplitude independent of its panning direction.

International patent application WO 2014/159272 A1 describes a method of synthesizing a multichannel audio signal to correlate an audio object with an apparent size.

Summary of the invention

An apparatus is described for reproducing a multichannel audio signal costing one or more sound objects in which each sound object is present in a plurality of channels, the apparatus comprising:

a predetermined listening zone provided within the apparatus;

a plurality of first speakers provided spaced around a first arc in front of the listening area, each of the first speakers directed towards the listening area and substantially equidistant from each other;

a plurality of second speakers provided spaced around a second arc behind the listening area, each of the second speakers being directed towards the listening area; an amplifier arranged to produce an amplified signal from each channel in the audio signal, each amplified signal being provided with a corresponding first or second speaker;

whereby each sound object is reproduced by one or more speakers in such a way that the SPL at a spaced point of the device is smaller than the SPL in the listening area.

Preferably, the SPL at a spaced point of the apparatus the same distance as each first speaker is spaced from the listening zone is 15 dB less than the SPL in the listening zone.

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Preferably, the number of first and second speakers is at least 13, the number of first speakers being greater than the number of second speakers.

Preferably, the multichannel audio signal is produced by the method below.

According to a first aspect of the invention there is provided a method for producing a multichannel audio signal from one or more sound object signals, comprising:

For each sound object signal:

Produce a plurality of decorrelated width signals from the sound object signal by doubling a part of the sound object signal and applying gains to each of the duplicate signals, in which the applied gains follow a Gaussian distribution;

Process the odd plurality of width signals to produce a plurality of panoramic signals, each panoramic signal being correlated with a channel;

For each channel in the audio signal, combine the panoramic signals of each sound object for that channel.

Preferably, the method further comprises the stage of normalization of the applied gains such that the amplitude of the sum of the width signals is equal to the amplitude of the part of the sound object signal.

Preferably, the Gaussian distribution follows a standard deviation configurable by a user.

Preferably, the production stage of a plurality of decorrelated width signals further comprises processing each sound object signal using a transition filter to produce a low frequency part and a high frequency part, the plurality of decorrelated width signals being produced from the high frequency part.

Preferably, an odd plurality of decorrelated width signals is produced, in which the low frequency portion is applied to a central signal of the odd plurality of decorrelated width signals.

Preferably, the method further comprises processing each sound object signal to produce a depth corrected signal and produce the plurality of decorrelated width signals from the depth corrected signal.

Preferably, each sound object signal is processed to produce two depth corrected signals, a direct signal and a reverberant signal, in which the plurality of decorrelated width signals are produced from the direct signal, and in which the Reverberant signal is processed to produce a plurality of decorrelated reverberant output signals, with each decorrelated reverberant output signal correlated with at least one channel in the audio signal.

Brief description of the figures

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a top view, partially cut away, of an apparatus for reproducing a multichannel audio signal;

Figure 2 is a rear perspective view of the apparatus in Figure 1; Figure 3 is a front perspective view of the apparatus in Figure 1;

Figure 4 shows room sound pressure levels (SPL) when the apparatus of Figure 1 is in use;

Figure 5 shows comparable room SPLs using conventional stereo speakers and audio system;

Figure 6 shows comparable room SPLs using conventional multi-channel speakers and audio system; Y

Figure 7 is a signal processing diagram showing a method of producing a multichannel audio signal according to an embodiment of the invention.

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Description of preferred embodiments

Figures 1 to 3 show an apparatus 10 for reproducing a multichannel audio signal. The apparatus 10 comprises a plurality of first speakers 12 provided spaced around a first arc 14. Each of the first speakers 12 is directed towards a listening area 16 provided within the apparatus 10. The first speakers 12 are each preferably substantially equidistant. from the listening area 16. The first arc 14 is preferably circular as shown in the drawings; however, other elliptic or arcuate curves can also be used.

A plurality of second speakers 18 are provided spaced around a second arc 20. Each of the second speakers 18 is directed towards the listening zone 16.

A listener 22 is shown in Figure 1 in the listening zone 16 which is directed towards the first speakers 12. Throughout the entire specification, the terms 'front' and 'behind' are used in relation to the zone of listen 16 according to the orientation of the listener 22 shown in Figure 1.

As seen in Figure 1, the first speakers 12 are placed in front of the listening area 16 and surround the front 180 ° of the listening area 16. The second speakers 18 are placed behind the listening area 16. In the embodiment , thirteen (13) first speakers 12 and five (5) second speakers 18 are used, although other quantities may be used. However, it is preferred that the number of first and second speakers be at least thirteen.

Two low frequency controllers 24 are provided, on each side and behind the listening zone 16 in an enclosure 26. The low frequency controllers 24 are configured as bass sound speakers.

An amplifier 28 produces amplified signals from each channel in the audio signal. Preferably, the audio signal has a separate channel for each speaker 12, 18 and 24. Therefore, the amplifier 28 provides a separate amplified signal to each speaker and the bass sound speakers. The amplifier 28 is housed behind the listening zone 16 in the enclosure 26. The term amplifier 28 encompasses a multichannel amplifier, multiple single channel amplifiers or a combination of both. Class D amplifiers are preferred for efficiency although other classes may be used.

The apparatus 10 has a base 30 on which the enclosure 26 is mounted. Each first speaker 12 is provided in an enclosure 32 mounted on the base 30. Adjacent enclosures 32 are connected through plates 34 extending between their upper surfaces. When mounted in this manner, the enclosures 32 form a continuous arc.

The multichannel audio signal consists of one or more sound objects. Each sound object is present in a plurality of channels of the audio signal as will be described in more detail below.

When the audio signal is reproduced by the apparatus 10, each sound object is reproduced by one or more speakers 12, 18. The sound of each speaker converges in the listening area 16. Since each speaker 12 is substantially equidistant from the listening zone 16, sounds from adjacent speakers 12 that reproduce a sound object will be constructively added to the listening zone 16.

When the apparatus 10 reproduces the audio signal, the SPL at a spaced point of the apparatus 10 is smaller than the SPL in the listening zone 16. Two factors contribute to this effect. First, the listening zone 16 is substantially equidistant from the speakers 12 such that their acoustic outputs are combined within the listening zone 16, while in other locations there will be different path lengths from each speaker resulting in destructive interference. Secondly, the loudspeakers are located near and face the listening area 16, while outside the apparatus 10 the average distance to the loudspeakers increases with the increasing distance from the apparatus, resulting in a reduced SPL.

Figures 4 to 6 show the results of SPL modeling in a 50 m2 room. In each of these figures, the model was established to produce a 125 dB SPL in the listening area and then the SPL was calculated throughout the room.

Figure 4 shows the SPL using the apparatus 10, in which the SPL on the walls of the room is at least 10 dB and up to 15-20 dB smaller than the listening area. Figure 5 shows the SPL using a traditional stereo layout. The SPL is higher in this arrangement in the intermediate vicinity of the speakers and adjacent walls. Figure 6 shows the SPL in typical multichannel systems with speakers on the periphery of the room. As shown, the SPL throughout the room and walls is relatively uniform.

The production of conventional audio signals involves the provision of monaural tracks, each track representing a sound object; Such tracks are also referred to as sound object signals. For a studio recording, there will be a track for each instrument and vocal singer. The sound engineer has

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these tracks, adjusting relative amplitudes. The tracks are then mixed and reduced to the number of channels using amplitude planning techniques.

The preferred method of producing an audio signal according to the embodiment involves three process steps applied to the track for each sound object - depth, width and panoramic - described below with reference to Figure 7.

Depth:

Each track, or sound object signal, is filtered through a second-order low-pass IIR filter 102, a low-order second-order IIR filter 104 and a high-order second-order IIR filter 106. These filters 102, 104 and 106 are applied to represent frequency variations that occur when the distance to a sound source increases. A gain stage 108, provided at the output of the filter 106, produces two output signals corrected in depth, referred to as direct and reverberant signals.

Examples of filters 102, 104 and 106 and gain stage 108 are given below for a depth parameter d having a value between 0 and 1, in which 0 is near the listener and 1 is far:

The filter 102 can be a low-pass filter of 2nd order of Butterworth with a cutoff frequency fc, in which fc =

20 kHz if d <= 0.2, and fc = 20 kHz - 15 kHz * (d - 0.2) / 0.8 if d> 0.2.

The filter 104 may be a low-order second-order IIR filter with a cutoff frequency of 80 Hz, Q =

0.5, and gain (dB) = 3.0 * (1.0 - 5 * d) 2 if d <= 0.2, and gain (dB) = -6.0 * ((d - 0.2 ) / 0.8) 2 if d> 0.2.

Filter 106 may be a high-order second-order IIR filter with a cutoff frequency of 16 kHz, Q =

0.5, and gain (dB) = 6.0 * (1.0 - 5 * d) 2 if d <= 0.2, and gain (dB) = 0.0 if d> 0.2.

The gain stage 108 may be a simple gain control in which gain (dB) = 3.0 * (1.0-5 * d) 2 if d

<= 0.2, and gain (dB) = -12.0 * ((d - 0.2) / 0.8) 2 if d> 0.2.

It should be appreciated that the above values are an example only and other values may be used.

The direct signal is passed to the width stage described below. The reverberant signal is processed using an acoustic space simulator 110. The simulator 110 adds a configurable amount of reverberation. The balancing of amplitudes of the direct and reverberant signals, for example in the gain stage 108, provides an additional sense of depth. Simulator 110 uses an algorithm of 1 input, n input. The n inputs have similar energy content, but they are decorated using feedback delay networks with different time constants for each output.

The decorrelated nature of the n outputs enables them to be reproduced by adjacent speakers without affecting the location of the listener 22 of the sound object (which is located by the direct signal), while contributing to focus the acoustic energy in the area of Listen 16 and provide a sense of depth. Typically, n <13 and the n outputs can be correlated with all channels in the audio signal, several of them being powered by the same output. Alternatively, the n outputs can be correlated with a subset of these channels using, for example, standard audio panning techniques.

Width:

The direct signal of the depth stage is introduced into a fourth order transition filter 112 that divides the signal into two bands: a low frequency part (LF) and a high frequency part (HF). The transition frequency of the filter 112 is chosen so that it is below the spatial sampling frequency fa = 2c / loudspeaker, where fa is the spatial sampling frequency, c is the speed of sound propagation in the air, and the speaker is the distance between the centers of two adjacent speakers. In the embodiment, the fa is approximately 500 Hz, but nothing prevents the use of a lower frequency.

The HF part of the signal is passed through k parallel gain stages 114, to produce k signals, with Figure 7 drawn for the case of k = 5. The gain stages 114 apply gains to each of the k signals following a Gaussian distribution, whose standard deviation is controlled by an adjustable width parameter. It is preferred that the gains of the gain stages 114 are standardized so that the sum of the outputs of the gain stages 114 does not show any amplitude deviation of the input signal HF. The greater the value of the width parameter, the more uniform the distribution of gains applied by the gain stages 114. This results in more control over the SPL outside the apparatus 10.

It is preferred that k be an odd number, so that the center of the k signals has an amplitude greater than the others of the k signals, which helps the listener 22 to locate the sound object. In other embodiments, values of k other than 5 may be used.

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Each of the k signals passes through one of k all-step FIR filters 116. Each FIR filter 116 alters the phase of the input signal with a spectral period T and a different initial phase compared to other FIR 116 filters to produce one of k width signals, shown in Figure 7 at 118. The k width signals are decorated in a cycle due to the effect of filters 116. Phase oscillation patterns such as sinusoids, as well as other oscillation patterns of phase.

The effect of the width processing stage is to produce k width signals with relative phase properties to allow reproduction in adjacent speakers of the apparatus 10, without creating frequency cancellations in the listening zone 16.

Figure 7 shows the part LF that adds to the central signal of the k signals. In other embodiments, the LF part could be applied to more than one of the k signals or follow the same gain / pan distribution as the HF part described above.

Panoramic:

The k width signals are each passed through a second-order IIR low equalization filter 120 and the gain stage 122 to produce k panoramic signals. Filter 120 provides a low frequency gain correction that reduces the change in the tone of a sound object when panned through speakers 12, 18. Typically, the gain of filter 120 is -3 dB when an object is equidistant to its two closest speakers.

Next, standard amplitude planning techniques are used to correlate the k panoramic signals with channels in the audio signal. The k panoramic signals are panned with an angular stage corresponding to the angular distance between speakers 12, 18 depending on the location of the sound object. This results in a set of signals, in k or k + 1 of the channels in the audio signal, with similar energy content but phase-related decor. This helps focus the acoustic energy in the listening area. The ability of the listener to locate the sound object is not affected: the listener will determine the location of a sound object based on the apparent highest sound source; the decorrelated signals on each side of the highest signal for each sound object so as not to affect the location of the listener of the sound object since the related decor does not have an apparent location for a listener.

The above processing is performed for each sound object and the outputs are combined so that the channel produces the multichannel audio signal. This processing technique provides a sound scenario with superior three-dimensionality, improved user capacity to accurately locate each sound object, while maintaining precise control of how acoustic energy propagates outside the apparatus.

Claims (7)

5 10 fifteen twenty 25 30 35 40 1. A method of producing a multichannel audio signal from one or more sound object signals, comprising: For each sound object signal: Produce a plurality of decorrelated width signals from the sound object signal by doubling a part of the sound object signal; Process the plurality of width signals to produce a plurality of panoramic signals, each panoramic signal being correlated with at least one channel; For each channel in the audio signal, combine the panoramic signals of each sound object for that channel; characterized in that for each sound object signal, the plurality of decorrelated width signals is produced by applying gains to each of the duplicate signals, in which the applied gains follow a Gauss distribution. 2. The method of claim 1, further comprising the step of normalizing the gains applied in such a way that the amplitude of the sum of the width signals is equal to the amplitude of the part of the sound object signal. 3. The method of claim 1 or 2, wherein the Gaussian distribution follows a standard deviation configurable by a user. 4. The method of claim 1, wherein the step of producing a plurality of decorrelated width signals further comprises processing each sound object signal using a transition filter to produce a low frequency part and a frequency part high, the plurality of decorrelated width signals produced from the high frequency part. 5. The method of claim 4, wherein for each audio object signal an odd plurality of decorrelated width signals is produced, wherein the low frequency portion is applied to a central signal having the greatest amplitude of the odd plurality of decorrelated width signals. 6. The method of claim 1, further comprising processing each sound object signal to produce a depth corrected signal and producing the plurality of decorrelated width signals from the depth corrected signal. 7. The method of claim 6, wherein each sound object signal is processed to produce two depth corrected signals, a direct signal and a reverberant signal, in which the plurality of decorrelated width signals are produced from of the direct signal, and in which the reverberant signal is processed to produce a plurality of decorrelated reverberant output signals having similar energy content, each decorrelated reverberant output signal correlated with at least one channel in the audio signal.